Evidence for Wave Heating of the Quiet Sun Corona

We have measured the energy and dissipation of Alfvenic waves in the quiet Sun. A magnetic field was used to infer the location and orientation of the magnetic field lines along which the waves are expected to travel. The waves were measured using spectral lines to infer the wave amplitude. The waves cause a non-thermal broadening of the spectral lines, which can be expressed as a non-thermal velocity v_nt. By combining the spectroscopic measurements with this magnetic field model we were able to trace the variation of v_nt along the magnetic field. At the footpoints of the quiet Sun loops we find that waves inject an energy flux in the range of 1.2-5.2 x 10^5 erg cm^-2 s^-1. At the minimum of this range, this amounts to more than 80% of the energy needed to heat the quiet Sun. We also find that these waves are dissipated over a region centered on the top of the loops. The position along the loop where the damping begins is strongly correlated with the length of the loop, implying that the damping mechanism depends on the global loop properties rather than on local collisional dissipation.Comment: Submitted to the Astrophysical Journa

Inferring the Coronal Density Irregularity from EUV Spectra

Understanding the density structure of the solar corona is important for modeling both coronal heating and the solar wind. Direct measurements are difficult because of line-of-sight integration and possible unresolved structures. We present a new method for quantifying such structure using density-sensitive EUV line intensities to derive a density irregularity parameter, a relative measure of the amount of structure along the line of sight. We also present a simple model to relate the inferred irregularities to physical quantities, such as the filling factor and density contrast. For quiet Sun regions and interplume regions of coronal holes, we find a density contrast of at least a factor of three to ten and corresponding filling factors of about 10-20%. Our results are in rough agreement with other estimates of the density structures in these regions. The irregularity diagnostic provides a useful relative measure of unresolved structure in various regions of the corona.Comment: Submitted to the Astrophysical Journa

Uncertainties in Dielectronic Recombination Rate Coefficients: Effects on Solar and Stellar Upper Atmosphere Abundance Determinations

We have investigated how the relative elemental abundances inferred from the solar upper atmosphere are affected by uncertainties in the dielectronic recombination (DR) rate coefficients used to analyze the spectra. We find that the inferred relative abundances can be up to a factor of ~5 smaller or ~1.6 times larger than those inferred using the currently recommended DR rate coefficients. We have also found a plausible set of variations to the DR rate coefficients which improve the inferred (and expected) isothermal nature of solar coronal observations at heights of >~ 50 arcsec off the solar limb. Our results can be used to help prioritize the enormous amount of DR data needed for modeling solar and stellar upper atmospheres. Based on the work here, our list of needed rate coefficients for DR onto specific isoelectronic sequences reads, in decreasing order of importance, as follows: O-like, C-like, Be-like, N-like, B-like, F-like, Li-like, He-like, and Ne-like. It is our hope that this work will help to motivate and prioritize future experimental and theoretical studies of DR.Comment: 33 pages, including 3 figures and 4 tables. To be published in Ap

Export Performance and Investment Behaviour of Firms in Ghana

From the theoretical literature a strong relationship between export performance and investment behaviour at the firm level is expected. A 2003 survey of 100 Ghanaian enterprises is used to analyse the factors that influence the investment and exporting behaviour of firms using a simultaneous equation model to allow for the endogeneity of investment and exporting. In addition, the different factors that influence the investment and export decisions in different sectors are investigated. However, no significant positive relationship between exporting and investment could be found. There seems rather to be a negative association, which might be explained by constraints in the access to capital. On the other hand there are several factors that work in the same direction, for example, younger firms, larger firms and more efficient firms are more likely to invest and more likely to export. --

Relative Abundance Measurements in Plumes and Interplumes

We present measurements of relative elemental abundances in plumes and interplumes. Plumes are bright, narrow structures in coronal holes that extend along open magnetic field lines far out into the corona. Previous work has found that in some coronal structures the abundances of elements with a low first ionization potential (FIP) < 10 eV are enhanced relative to their photospheric abundances. This coronal-to-photospheric abundance ratio, commonly called the FIP bias, is typically 1 for element with a high-FIP (> 10 eV). We have used EIS spectroscopic observations made on 2007 March 13 and 14 over an ~24 hour period to characterize abundance variations in plumes and interplumes. To assess their elemental composition, we have used a differential emission measure (DEM) analysis, which accounts for the thermal structure of the observed plasma. We have used lines from ions of iron, silicon, and sulfur. From these we have estimated the ratio of the iron and silicon FIP bias relative to that for sulfur. From the results, we have created FIP-bias-ratio maps. We find that the FIP-bias ratio is sometimes higher in plumes than in interplumes and that this enhancement can be time dependent. These results may help to identify whether plumes or interplumes contribute to the fast solar wind observed in situ and may also provides constraints on the formation and heating mechanisms of plumes.Comment: 21 pages; 3 tables; 12 figure

Evidence of Wave Damping at Low Heights in a Polar Coronal Hole

We have measured the widths of spectral lines from a polar coronal hole using the Extreme Ultraviolet Imaging Spectrometer onboard Hinode. Polar coronal holes are regions of open magnetic field and the source of the fast solar wind. We find that the line widths decrease at relatively low heights. Previous observations have attributed such decreases to systematic effects, but we find that such effects are too small to explain our results. We conclude that the line narrowing is real. The non-thermal line widths are believed to be proportional to the amplitude of Alfven waves propagating along these open field lines. Our results suggest that Alfven waves are damped at unexpectedly low heights in a polar coronal hole. We derive an estimate on the upper limit for the energy dissipated between 1.1 and 1.3 solar radii and find that it is enough to account for up to 70% of that required to heat the polar coronal hole and accelerate the solar wind.Comment: Accepted for publication in the Astrophysical Journal, April 201